1. Field of the Invention
The invention relates to a corrector, which is equipped with multipole elements in the form of electrical and/or magnetic quadrupole and octupole elements, comprising a straight optical axis for eliminating the chromatic aberration and the divergence aberration of particle-optical lenses.
2. Description of the Prior Art
Particle-optical systems are successfully used both in enlarging systems, such as the electron microscopes, and in diminishing systems, such as electron-projection lithography. Compared with light-optical systems, they have the advantage of a substantially higher resolution, this magnitude being all the more marked the smaller the wavelength of the beams used. Where electron beams are used, this means a gain in resolution of a factor of 104 compared to light.
A further improvement of the resolution of particle-optical systems requires a correction of the imagining lenses' own image aberrations. According to conventional nomenclature, these image aberrations are classified into:                Axial aberrations, which occur with the imaging of an axial point and are only dependent on the divergence angle. They are determined by the profile of the axial fundamental paths, (xα), (yβ), which emerge from an axial point on the object plane and lie in the x and y cross-sections.        Extra-axial aberrations, which are effective when an extra-axial image point is represented, and depend on the distance to the optical axis (and usually on the divergence angle) They are determined by the profile of the extra-axial fundamental paths (xγ), (yδ), which emerge from an axially remote point on the object plane and lie in the x and y cross-sections,        Chromatic aberrations, which occur with particles of different velocities, are further subdivided into axial and extra-axial chromatic aberrations and correspondingly depend on the divergence angle and/or on the distance from the optical axis.)        
For axial aberrations, which only depend on the divergence angle, the term divergence aberrations is also commonly used, and for the axial and extra-axial image aberrations together, to distinguish from chromatic aberrations, the term geometrical image aberrations is also used.
Considerable efforts have been made in the past to correct these image aberrations, in particular chromatic aberration (=1st order axial aberrations) and divergence aberration (=3rd order divergence aberrations). The best successes to date have been achieved with optical systems in which, instead of rotationally symmetrical fields, the fields of multipoles, in particular quadrupoles and octupoles, are used. Systems of this kind are generally given the name corrective or corrector.
In the prior art, correctives or correctors of different designs are known.
German Patent 42 04 512 A1 discloses an electron-optical corrective which is constructed of 6 quadrupole elements and 7 octupole elements, three quadrupoles and three octupoles being provided in each case before and after the centre plane of the system, and a further octupole being arranged in the centre plane. The system permits simultaneous correction of the axial and extra-axial chromatic aberration and of the spherical aberration. Its advantage lies in a simple construction, a relatively small number of multipole elements and a comparatively small overall length. A disadvantage of this system, however, is that in applications in the maximum resolution range, the electrical supply is subject to extremely high stability requirements, which could not be satisfied hitherto. Furthermore, the magnitude of the image field of an objective lens corrective in the present corrective is limited in a disadvantageous manner, since the 3rd order extra-axial image aberration of the lens with this system cannot be corrected.
A corrector for elimination of the chromatic aberration with purely electrical fields is presented in German Patent DE 199 26 927.0. The corrector proposed here has two corrective parts, which are arranged one behind the other along the optical axis and have in each case electrical quadrupole fields and superimposed octupole fields. The corrector permits a correction of the 1st order axial chromatic aberration and the 3rd order divergence aberration. The advantage of this corrector over that mentioned above is that the fields required for correction are electrostatic in nature and can therefore be adjusted precisely and, in particular, reproducibly. Because of the lack of correction of extra-axial aberrations, a high resolution can disadvantageously only be achieved in regions extremely close to the axis, so that the useful image-field size is greatly restricted.
Furthermore, a corrector is also known from German Patent 100 01 277.9, in which, in contrast to the method used here, the correction of image aberrations is carried out by means of a set of hexapoles, which are arranged alternately with circular lenses along the optical axis. In contrast to the correctors described above, that proposed here shows stigmatic intermediate images. This embodiment permits a correction of all 3rd order geometrical aberrations and makes comparatively low demands of the stability of the electrical supply. A considerable disadvantage of this solution, however, lines in the fact that the provided corrector does not permit correction of the image aberration. Furthermore, the circular lenses acting as transfer lenses generate additional axial chromatic aberrations. The stigmatic intermediate images lead, because of the Boersch effect occurring in this area, to an additional energy spread of the electrons. The corrector also disadvantageously has a comparatively large overall length and can therefore only be used with restrictions.